jfreechart显示不出图片，求大神解惑

xml配置如下
`
DisplayChart
org.jfree.chart.servlet.DisplayChart


DisplayChart
/servlet/DisplayChart

`
jsp内容如下

<%@ page contentType="text/html;charset=GBK"%>
<%@ page import="org.jfree.chart.ChartFactory,
org.jfree.chart.JFreeChart,
org.jfree.chart.plot.PlotOrientation,
org.jfree.chart.servlet.ServletUtilities,
org.jfree.data.category.DefaultCategoryDataset"%>
<%
DefaultCategoryDataset dataset = new DefaultCategoryDataset();
dataset.addValue(610, "Guangzhou", "Pig");
dataset.addValue(220, "Guangzhou", "Beef");
dataset.addValue(530, "Guangzhou", "Chicken");
dataset.addValue(340, "Guangzhou", "Fish");
JFreeChart chart = ChartFactory.createBarChart3D("肉类销量统计图",
"type",
"amount",
dataset,
PlotOrientation.VERTICAL,
false,
false,
false);
String filename = ServletUtilities.saveChartAsPNG(chart, 500, 300, null, session);
String graphURL = request.getContextPath() + "/DisplayChart?filename=" + filename;
%>
<img src="<%= graphURL %>" width=500 height=300 border=0 usemap="#<%= filename %>">

在浏览器访问得到一个x,但是在tomcattemp文件夹里面有表

求大神解惑